Chap. X.] THE LAPSE OF TIME. 57 



animals which propagate their kind much more slowly 

 than most of the lower animals, that they have formed 

 what well deserves to be called a new sub-breed. Few 

 men have attended with due care to any one strain for 

 more than half a century, so that a hundred years repre- 

 sents the work of two breeders in succession. It is not 

 to be supposed that species in a state of nature ever 

 change so quickly as domestic animals under the guid- 

 ance of methodical selection. The comparison would 

 be in every way fairer with the effects which follow 

 from unconscious selection, that is the preservation of 

 the most useful or beautiful animals, with no intention 

 of modifying the breed; but by this process of uncon- 

 scious selection, various breeds have been sensibly 

 changed in the course of two or three centuries. 



Species, however, probably change much more 

 slowly, and within the same country only a few change 

 at .the same time. This slowness follows from all the 

 inhabitants of the same country being already so well 

 adapted to each other, that new places in the polity of 

 nature do not occur until after long intervals, due to the 

 occurrence of physical changes of some kind, or through 

 the immigration of new forms. Moreover variations or 

 individual differences of the right nature, by which 

 some of the inhabitants might be better fitted to their 

 new places under the altered circumstances, would not 

 always occur at once. Unfortunately we have no means 

 of determining, according to the standard of years, how 

 long a period it takes to modify a species; but to the 

 subject of time we must return. 



